The present invention relates to a device for establishing an ambient temperature correction for the results of gas analyses performed with a long-path Fourier Transform Infrared (FTIR) spectrophotometer and to the use of this device to obtain accurate and reproducible values.
Absorption spectra may be used to determine the presence and amounts of infra-red absorbing gases present in the atmosphere at selected locations. Such spectra are generated by collecting radiation from the selected location and subjecting this background radiation to FTIR analysis. Devices for generating emission spectra from such background radiation are known. See, for example, U.S. Pat. Nos. 4,795,253 and 4,999,498.
When used with a remote infra-red source (i.e., bistatic mode), these known devices typically employ spectra obtained from surfaces where the FTIR is located (e.g., a roof surface near the instrument, the side of a shaded wall, a grassy lawn, a hillside, the base plate of the instrument, etc.) to correct spectra taken with long-path FTIR instruments. However, it has been found that when the FTIR analyzer is used at longer distances or with weaker sources, spectra from such backgrounds are inadequate and introduce unacceptable errors. Similarly a constant temperature reference surface would not adequately reflect the ambient temperature of the selected location because such a surface would not reflect changes in air temperature from night to day or from winter to summer. It would, therefore, be advantageous to have a reference surface which adequately represents the radiation characteristics of the column of air through which the long-path FTIR is looking.